Try chest and biceps or triceps, back with whatever arm muscle you don't use. Swap them round from week to week. Also, to build real mass, you're going to need to work shoulders and legs too. Try including compound exercises. Pull ups, squats, deadlifts, that kind of thing. Don't work the same muscles constantly, make sure each muscle group gets at least a good day of rest after a hard workout.

I wouldnt do it either way. Chest and back in one day is too much for one workout. You cant have a full on workout on one without sacrificing the energy you would be using for the next. So if you do back first, you are cheating your chest out of a quality workout. Or vice versa. I dont recommend doing two large muscle groups in the same workout. And if you do chest and then triceps or back and then biceps, you will have already taxed your arms before you get to them and will, again, not be able to commit a full workout. Try doing pulling muscles on one day and then pushing muscles another day, but do muscles that dont affect each other. For example, I do chest with calves. Both pushing muscles, but neither is used in working the other. here is an example of my training split: Day 1)-Shoulders and triceps Day 2)-Biceps, forearms, and hamstrings Day 3)-off Day 4)Quads Day 5)-off Day 6)-Back Day 7)-Chest and calves Day 8)-off as you can see, I train everything just once a week. That allows the muscles plenty of time to rest before being worked out again. Also, I make sure that I dont train a muscle that will have to be used to help me with another muscle for at least a couple of days. Like doing chest the day after you do triceps is not giving the triceps enough rest. Resting is when the muscles build up larger, so if you dont give them enough time to rest before you break them down again, you will actually lose size.